Emergency power units which supply electrical or hydraulic power for critical flight control systems on an aircraft, following a loss of main engine power, must come on line and begin producing power virtually instantaneously when they are needed. The same is true for power transmission or generation apparatus used in stationary emergency power generators which supply electricity to a computer or a hospital, for instance, in the event of a power outage in a municipal power grid. These systems must provide power virtually instantaneously despite the fact that they may have been sitting idle for extended periods of time at ambient temperatures of -20.degree. F. or below.
Such emergency power units often utilize a hydraulic clutch to couple an engine or other prime mover to a generator, a pump, or some other driven device. The clutch allows the driven device to be de-coupled from the engine during startup to reduce the load on the engine start system. The clutch also allows the engine to be rapidly de-coupled from the driven device for safety or other reasons. In some instances the clutch is further required to provide a controlled amount of slip to achieve a smooth engagement, or to compensate for rapid fluctuations in the load imposed on the engine by the driven device.
The hydraulic clutches in such emergency power units must engage controllably, responsively and reliably in order to perform their required functions. To achieve such performance from the clutch, however, it is necessary that the hydraulic system supplying fluid to the clutch be properly designed to allow rapid controlled response under adverse conditions.
A typical hydraulic system in these applications includes a pump or other source of pressurized fluid, a low pressure reservoir, a source of control signals, a fluid actuated device, such as a hydraulic clutch, and a fluid control valve. These components are connected to form a fluid circuit in which the valve receives a flow of pressurized fluid from the pump. The valve converts a portion of that flow of pressurized fluid into control flow or pressure that is then supplied to the fluid actuated device in response to control signals received from the source or control signals. The remaining small leakage portion of the flow of pressurized fluid passes through the valve and returns to the low pressure reservoir. Although the clutch control circuit can be a separate dedicated hydraulic system, it is often advantageous to combine the control circuit with other hydraulic circuits used for lubrication or cooling of parts of the overall apparatus in which the clutch control system operates. By combining the hydraulic circuits in this manner, a single pump and reservoir system may serve several hydraulic circuit loops, thereby reducing the complexity and cost of the hydraulic system.
The control circuits of such hydraulic systems must be capable of responding quickly when they are needed, despite the fact that the hydraulic fluid may have become highly viscous during a prolonged period of inoperation coupled with exposure to extremely cold ambient temperatures. Such highly viscous fluid does not flow readily, however, thus making it difficult to achieve rapid response. Furthermore, it is often difficult to provide a control valve that is capable of achieving and maintaining stable control at fluid temperatures which are significantly different than the normal operating temperature of the fluid.
To facilitate rapid response and to ensure stable control on startup, hydraulic systems have sometimes included means for heating the fluid prior to engaging the clutch. In some such systems, an auxiliary heater is utilized to warm the fluid in the reservoir. To achieve instantaneous actuation and stability with this type of system it is generally necessary that the heating device be operated continuously to maintain the fluid within a temperature range known to provide acceptable performance. The wasted energy costs for this type of system can be substantial if the system must be maintained in a ready state while standing idle for extended periods of time.
Alternatively, a heating device that is capable of rapidly raising the temperature of the fluid to operating temperature "on demand" just prior to startup can be employed, with actual engagement of the clutch being delayed following an engagement command until the fluid has reached operating temperature. Obviously, for an emergency power unit such a delay is undesirable. In an aircraft emergency power unit, for instance, it is sometimes a requirement that the unit be fully operational and producing controlled power within 30 seconds of receiving a start command.
While any delay is undesirable, the "on-demand" approach is often preferred for practical reasons such as reducing the operating cost or complexity of the hydraulic system. In addition, and particularly where the prime mover is an engine, various waste heat sources such as frictional and dynamic pressure losses in the hydraulic circuit, exhaust gases, or an engine cooling circuit are often available for on demand heating of the fluid in the hydraulic circuit without the need for external energy input. A temperature responsive valve is often utilized in this type of system to provide heat only during the "on demand" heating period.
Although these "on-demand" heating approaches are acceptable in some instances, they are not entirely satisfactory for emergency power units driven by air cooled gas turbine engines. In these engines, there is no separate liquid or air cooling circuit. Cooling is provided by the air flowing through the engine from the inlet to the exhaust. The exhaust gases are typically very hot, i.e. over 1000.degree. F., and therefore present substantial challenges in designing the ducting, control valves, and heat exchangers which would be required to utilize this heat. Specifically, the extremely high temperature of the exhaust gas rules out the use of lightweight materials such as aluminum for components exposed to the exhaust gases. The additional weight penalty incurred thus generally precludes the use of exhaust gases for cooling in aircraft mounted power units.
Because of these problems, many prior gas turbine engine driven power units have resorted to simply allowing the fluid in the hydraulic system to circulate and be warmed by frictional and dynamic pressure losses in the circuit for some period of time prior to carrying out a command to engage the clutch. For start-up at temperatures in the range of -20.degree. F. and below, this has resulted in unacceptable delays in bringing the power transmission or generation unit on line. This is particularly true where there is a shared hydraulic system providing fluid to other control, lubrication, or cooling circuits in addition to the clutch control circuit, and all of the fluid circulating in the hydraulic system must be warmed to a temperature at which rapid actuation and stable control of the clutch can be achieved.
It is an object of our invention, therefore, to provide an improved hydraulic clutch. It is also an object of our invention to provide an improved hydraulic system that is capable of effectively and efficiently heating at least a portion of the fluid in the clutch control loop to a temperature at which rapid actuation and stable control can be achieved in a significantly shorter period of time than can be accomplished in prior hydraulic systems using waste heat to warm a fluid in an on-demand heating approach. Additional objects of our invention include providing:
1. a gas turbine engine powered auxiliary power unit (APU) capable of providing a controlled power output in less than 30 seconds after receiving a start command when the APU has been exposed for an extended period of time to ambient temperatures of -20.degree. F.; PA1 2. a hydraulic control system capable of engaging and providing stable control of a hydraulic clutch in less than 30 seconds after prolonged exposure to ambient temperatures of -20.degree. F.; PA1 3. an on-demand heating system which is applicable to hydraulic systems having either single or multiple branches and capable of heating at least a portion of the fluid in the system to a temperature at which a control circuit of the hydraulic system can engage and achieve stable control of a fluid actuated device, such as a hydraulic clutch, in less than 30 seconds after prolonged exposure to ambient temperatures of -20.degree. F.; and PA1 4. a straightforward and inexpensive arrangement for producing a hydraulic system meeting the above stated objects.